Bowed rotor start mitigation in a gas turbine engine using aircraft-derived parameters

ABSTRACT

A bowed rotor start mitigation system for a gas turbine engine of an aircraft is provided. The bowed rotor start mitigation system includes a motoring system and a controller coupled to the motoring system and an aircraft communication bus. The controller is configured to determine at least one inferred engine operating thermal parameter from at least one aircraft-based parameter received on the aircraft communication bus. The motoring system is controlled to drive rotation of a starting spool of the gas turbine engine below an engine idle speed based on determining that the at least one inferred engine operating thermal parameter is within a preselected threshold.

BACKGROUND

This disclosure relates to gas turbine engines, and more particularly toan apparatus, system and method for mitigating a bowed rotor startcondition in a gas turbine engine using aircraft-derived parameters.

Gas turbine engines are used in numerous applications one of which isfor providing thrust to an airplane. When the gas turbine engine of anairplane has been shut off for example, after an airplane has landed atan airport, the engine is hot and due to heat rise, the upper portionsof the engine will be hotter than lower portions of the engine. Whenthis occurs thermal expansion may cause deflection of components of theengine which may result in a “bowed rotor” condition. If a gas turbineengine is in such a “bowed rotor” condition it is undesirable to restartor start the engine.

Accordingly, it is desirable to provide a method and/or apparatus fordetecting and preventing a “bowed rotor” condition.

BRIEF DESCRIPTION

In an embodiment, a bowed rotor start mitigation system for a gasturbine engine of an aircraft is provided. The bowed rotor startmitigation system includes a motoring system and a controller coupled tothe motoring system and an aircraft communication bus. The controller isconfigured to determine at least one inferred engine operating thermalparameter from at least one aircraft-based parameter received on theaircraft communication bus. The motoring system is controlled to driverotation of a starting spool of the gas turbine engine below an engineidle speed based on determining that the at least one inferred engineoperating thermal parameter is within a preselected threshold.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the at least one inferred engine operating thermalparameter is selected from data describing a history of the aircraftbefore an engine shutdown.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where a time since the engine shutdown is used to modify a timeof rotation of the starting spool by the motoring system.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the controller transmits a notification on the aircraftcommunication bus that the gas turbine engine is being conditioned forstarting.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the controller is configured to abort rotation by themotoring system when a safety condition is detected.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the motoring system is configured to apply a brakingtorque to the starting spool to abort rotation of the starting spool.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude a rotation sensor for detecting rotary motion of the startingspool, and the controller is further coupled to the rotation sensor.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the controller is configured to remove power from themotoring system based on detecting rotary motion of the starting spoolabove a target speed.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where a vibration monitor is used to set a maintenance flag.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the at least one inferred engine operating thermalparameter includes an engine temperature value, and the at least oneaircraft-based parameter received on the aircraft communication busincludes a throttle lever angle profile.

According to an embodiment, a method of bowed rotor start mitigation fora gas turbine engine of an aircraft is provided. The method includesdetermining at least one inferred engine operating thermal parameterfrom at least one aircraft-based parameter received on an aircraftcommunication bus. The method also includes controlling a motoringsystem to drive rotation of a starting spool of the gas turbine enginebelow an engine idle speed based on determining that the at least oneinferred engine operating thermal parameter is within a preselectedthreshold.

A technical effect of the apparatus, systems and methods is achieved byusing a start sequence for a gas turbine engine as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the present disclosure isparticularly pointed out and distinctly claimed in the claims at theconclusion of the specification. The foregoing and other features, andadvantages of the present disclosure are apparent from the followingdetailed description taken in conjunction with the accompanying drawingsin which:

FIG. 1 is a cross-sectional view of a gas turbine engine;

FIG. 2 is a schematic illustration of a starting system for a gasturbine engine in accordance with an embodiment of the disclosure;

FIG. 3 is a schematic illustration of a starting system for a gasturbine engine in accordance with another embodiment of the disclosure;

FIG. 4 is a block diagram of a system for bowed rotor start mitigationin accordance with an embodiment of the disclosure;

FIG. 5 is a flow chart illustrating a method of bowed rotor startmitigation of a gas turbine engine in accordance with an embodiment ofthe disclosure;

FIG. 6 is a graph illustrating a bowed rotor risk score with respect totime in accordance with an embodiment of the disclosure;

FIG. 7 is a graph illustrating a normal or cooled engine start versus amodified engine start in accordance with an embodiment of thedisclosure;

FIG. 8 is a graph illustrating examples of various vibration levelprofiles of an engine in accordance with an embodiment of thedisclosure;

FIG. 9 is a schematic illustration of a high spool gas path with astraddle-mounted spool in accordance with an embodiment of thedisclosure;

FIG. 10 is a schematic illustration of a high spool gas path with anoverhung spool in accordance with an embodiment of the disclosure;

FIG. 11 is a graph illustrating commanded starter valve opening withrespect to time in accordance with an embodiment of the disclosure; and

FIG. 12 is a graph illustrating a target rotor speed profile of a drymotoring profile and an actual rotor speed versus time in accordancewith an embodiment of the disclosure.

DETAILED DESCRIPTION

Various embodiments of the present disclosure are related to a bowedrotor start mitigation system in a gas turbine engine. Embodiments caninclude a mission risk factor model used to estimate heat stored in anengine core at shutdown and identify a risk of a bowed rotor in the gasturbine engine based on one or more aircraft parameters. Informationfrom the mission risk factor model can be used by a bowed rotor startrisk model to calculate a bowed rotor risk parameter. As used herein theterm “model” may be referred to in one non-limiting manner as a processfor representing real world values, measurements or conditions throughthe use of a computer program or algorithm. The bowed rotor riskparameter may be used to take a control action to mitigate the risk ofstarting the gas turbine engine with a bowed rotor. The control actioncan include performing dry motoring as further described herein.

During dry motoring, a starter valve can be actively adjusted to deliverair pressure from an air supply to an engine starting system thatcontrols starting rotor speed. Dry motoring may be performed by runningan engine starting system at a lower speed with a longer duration thantypically used for engine starting while dynamically adjusting thestarter valve to maintain the rotor speed and/or follow a dry motoringprofile. Some embodiments increase the rotor speed of the starting spoolto approach a critical rotor speed gradually and as thermal distortionis decreased they then accelerate beyond the critical rotor speed tocomplete the engine starting process. The critical rotor speed refers toa major resonance speed where, if the temperatures are unhomogenized,the combination of a bowed rotor and similarly bowed casing and theresonance would lead to high amplitude oscillation in the rotor and highrubbing of blade tips on one side of the rotor, especially in the highpressure compressor if the rotor is straddle-mounted.

In some embodiments, a targeted rotor speed profile of the dry motoringprofile can be adjusted as dry motoring is performed. As one example, ifexcessive vibration is detected as the rotor speed rises and approachesbut remains well below the critical rotor speed, then the rate of rotorspeed increases scheduled in the dry motoring profile can be reduced(i.e., a shallower slope) to extend the dry motoring time. Similarly, ifvibration levels are observed below an expected minimum vibration levelas the rotor speed increases, the dry motoring profile can be adjustedto a higher rate of rotor speed increases to reduce the dry motoringtime.

A full authority digital engine control (FADEC) system or other systemmay send a message to the cockpit to extend time at idle to cool downthe rotor prior to shut down. If the engine is in a ground test or in atest stand, a message can be sent to the test stand or cockpit based onthe control-calculated risk of a bowed rotor. A test stand crew can bealerted regarding a requirement to bring the starting spool of theengine to a speed below the known resonance speed of the rotor in orderto homogenize the temperature of the rotor and the casings about therotor which also are distorted by temperature non-uniformity.

Monitoring of vibration signatures during the engine starting sequencecan also or separately be used to assess the risk that a bowed rotorstart has occurred due to some system malfunction and then directmaintenance, for instance, in the case of suspected outer air seal rubespecially in the high compressor. Vibration data for the engine canalso be monitored after bowed rotor mitigation is performed during anengine start sequence to confirm success of bowed rotor mitigation. Ifbowed rotor mitigation is unsuccessful or determined to be incomplete bythe FADEC, resulting metrics (e.g., time, date, global positioningsatellite (GPS) coordinates, vibration level vs. time, etc.) of theattempted bowed rotor mitigation can be recorded and/or transmitted todirect maintenance

According to various embodiments, there are a number of optionsavailable to mitigate a bowed rotor start depending on a presentoperating state of the gas turbine engine, instrumentation, andmonitoring systems implemented. For example, a control-calculated riskof a bowed rotor can be computed as a bowed rotor risk parameter asfurther described herein and used to trigger a cockpit message or teststand message to extend a time period to run at idle power prior toengine shutdown as a pre-shutdown mitigation. Alternatively, the bowedrotor risk parameter can trigger a request message or automatedinitiation of a dry motoring sequence prior to engine start. During adry motoring sequence, a starter air pressure valve can be modulated tolimit high rotor speed below high spool resonance speed and prevent rubduring dry motoring operation. The bowed rotor risk parameter can alsobe used to limit dry motoring duration to reduce the impact on airstarter turbine life. The monitoring of vibration signatures during theentire engine starting sequence can also or separately be used to assessthe risk of a bowed rotor start and direct maintenance, for instance, inthe case of suspected outer air seal rub especially in the highcompressor.

Rather than using sensed parameters of an engine system, embodiments useparameters at the aircraft level, e.g., as received on an aircraftcommunication bus, to derive parameters at the engine level, such asengine core temperature. For example, a profile of a throttle leverangle (TLA) parameter history can indicate a history of fuel flowrequested over a period of time which may be used to infer an enginecore temperature. By determining an amount of elapsed time from the lastreceived valid set of aircraft parameters in combination with a knownengine state, e.g., engine shutdown, a minimum dry motoring time can becalculated.

Referring now to FIG. 1, a schematic illustration of a gas turbineengine 10 is provided. The gas turbine engine 10 has among othercomponents a fan through which ambient air is propelled into the enginehousing, a compressor for pressurizing the air received from the fan anda combustor wherein the compressed air is mixed with fuel and ignitedfor generating combustion gases. The gas turbine engine 10 furthercomprises a turbine section for extracting energy from the combustiongases. Fuel is injected into the combustor of the gas turbine engine 10for mixing with the compressed air from the compressor and ignition ofthe resultant mixture. The fan, compressor, combustor, and turbine aretypically all concentric about a central longitudinal axis of the gasturbine engine 10. Thus, thermal deflection of the components of the gasturbine engine 10 may create the aforementioned bowing or “bowed rotor”condition along the common central longitudinal axis of the gas turbineengine 10 and thus it is desirable to clear or remove the bowedcondition prior to the starting or restarting of the gas turbine engine10.

FIG. 1 schematically illustrates a gas turbine engine 10 that can beused to power an aircraft, for example. The gas turbine engine 10 isdisclosed herein as a multi-spool turbofan that generally incorporates afan section 22, a compressor section 24, a combustor section 26 and aturbine section 28. The fan section 22 drives air along a bypassflowpath while the compressor section 24 drives air along a coreflowpath for compression and communication into the combustor section 26then expansion through the turbine section 28. Although depicted as aturbofan gas turbine engine in the disclosed non-limiting embodimentwith two turbines and is sometimes referred to as a two spool engine, itshould be understood that the concepts described herein are not limitedto use with turbofans as the teachings may be applied to other types ofturbine engines including three-spool architectures. In both of thesearchitectures the starting spool is that spool that is located aroundthe combustor, meaning the compressor part of the starting spool isflowing directly into the combustor and the combustor flows directlyinto the turbine section.

The engine 10 generally includes a low speed spool 30 and a high speedspool 32 mounted for rotation about an engine central longitudinal axisA relative to an engine static structure 36 via several bearing systems38. It should be understood that various bearing systems 38 at variouslocations may alternatively or additionally be provided.

The low speed spool 30 generally includes an inner shaft 40 thatinterconnects a fan 42, a low pressure compressor 44 and a low pressureturbine 46. The inner shaft 40 is connected to the fan 42 through ageared architecture 48 to drive the fan 42 at a lower speed than the lowspeed spool 30 in the example of FIG. 1. The high speed spool 32includes an outer shaft 50 that interconnects a high pressure compressor52 and high pressure turbine 54. A combustor 56 is arranged between thehigh pressure compressor 52 and the high pressure turbine 54. Amid-turbine frame 57 of the engine static structure 36 is arrangedgenerally between the high pressure turbine 54 and the low pressureturbine 46. The mid-turbine frame 57 further supports bearing systems 38in the turbine section 28. The inner shaft 40 and the outer shaft 50 areconcentric and rotate via bearing systems 38 about the engine centrallongitudinal axis A which is collinear with their longitudinal axes.

The core airflow is compressed by the low pressure compressor 44 thenthe high pressure compressor 52, mixed and burned with fuel in thecombustor 56, then expanded over the high pressure turbine 54 and lowpressure turbine 46. The mid-turbine frame 57 includes airfoils 59 whichare in the core airflow path. The turbines 46, 54 rotationally drive therespective low speed spool 30 and high speed spool 32 in response to theexpansion.

A number of stations for temperature and pressure are defined withrespect to the gas turbine engine 10 according to conventionalnomenclature. Station 2 is at an inlet of low pressure compressor 44having a temperature T2 and a pressure P2. Station 2.5 is at an exit ofthe low pressure compressor 44 having a temperature T2.5 and a pressureP2.5. Station 3 is at an inlet of the combustor 56 having a temperatureT3 and a pressure P3. Station 4 is at an exit of the combustor 56 havinga temperature T4 and a pressure P4. Station 4.5 is at an exit of thehigh pressure turbine 54 having a temperature T4.5 and a pressure P4.5.Station 5 is at an exit of the low pressure turbine 46 having atemperature T5 and a pressure P5. Embodiments use aircraft-derivedparameters rather than measured parameters at the stations depicted inFIG. 1 for bowed rotor mitigation.

Although FIG. 1 depicts one example configuration, it will be understoodthat embodiments as described herein can cover a wide range ofconfigurations. For example, embodiments may be implemented in aconfiguration that is described as a “straddle-mounted” spool 32A ofFIG. 9. This configuration places two bearing compartments 37A and 39A(which may include a ball bearing and a roller bearing respectively),outside of the plane of most of the compressor disks of high pressurecompressor 52A and at outside at least one of the turbine disks of highpressure turbine 54A. In contrast with a straddle-mounted spoolarrangement, other embodiments may be implemented using an over-hungmounted spool 32B as depicted in FIG. 10. In over-hung mounted spool32B, a bearing compartment 37B is located forward of the first turbinedisk of high pressure turbine 54B such that the high pressure turbine54B is overhung, and it is physically located aft of its main supportingstructure. The use of straddle-mounted spools has advantages anddisadvantages in the design of a gas turbine, but one characteristic ofthe straddle-mounted design is that the span between the bearingcompartments 37A and 39A is long, making the amplitude of the high spotof a bowed rotor greater and the resonance speed that cannot betransited prior to temperature homogenization is lower. For any thrustrating, the straddle mounted arrangement, such as straddle-mounted spool32A, gives Lsupport/Dhpt values that are higher, and the overhungmounted arrangement, such as overhung spool 32B, can be as much as 60%of the straddle-mounted Lsupport/Dhpt. Lsupport is the distance betweenbearings (e.g., between bearing compartments 37A and 39A or betweenbearing compartments 37B and 39B), and Dhpt is the diameter of the lastblade of the high pressure turbine (e.g., high pressure turbine 54A orhigh pressure turbine 54B). As one example, a straddle-mounted enginestarting spool, such as straddle-mounted spool 32A, with a rollerbearing at bearing compartment 39A located aft of the high pressureturbine 54A may be more vulnerable to bowed rotor problems since theLsupport/Dhpt ranges from 1.9 to 5.6. FIGS. 9 and 10 also illustrate astarter 120 interfacing via a tower shaft 55 with the straddle-mountedspool 32A proximate high compressor 52A and interfacing via tower shaft55 with the overhung mounted spool 32B proximate high compressor 52B aspart of a starting system.

Turning now to FIG. 2, a schematic of a starting system 100 for the gasturbine engine 10 of FIG. 1 is depicted according to an embodiment. Thestarting system 100 is also referred to generally as a gas turbineengine system. In the example of FIG. 2, the starting system 100includes a controller 102 which may be an electronic engine control,such as a dual-channel FADEC, and/or engine health monitoring unit. Inan embodiment, the controller 102 may include memory to storeinstructions that are executed by one or more processors. The executableinstructions may be stored or organized in any manner and at any levelof abstraction, such as in connection with a controlling and/ormonitoring operation of the engine 10 of FIG. 1. The one or moreprocessors can be any type of central processing unit (CPU), including ageneral purpose processor, a digital signal processor (DSP), amicrocontroller, an application specific integrated circuit (ASIC), afield programmable gate array (FPGA), or the like. Also, in embodiments,the memory may include random access memory (RAM), read only memory(ROM), or other electronic, optical, magnetic, or any other computerreadable medium onto which is stored data and control algorithms in anon-transitory form.

The starting system 100 can also include a data storage unit (DSU) 104that retains data between shutdowns of the gas turbine engine 10 ofFIG. 1. The DSU 104 includes non-volatile memory and retains databetween cycling of power to the controller 102 and DSU 104. An aircraftcommunication bus 106 can include an aircraft-level and/or test standcommunication bus to interface with aircraft controls, e.g., a cockpit,various onboard computer systems, and/or a test stand. Based ondetecting a bowed rotor start risk, the controller 102 can send arequest for extended idle operation on the aircraft communication bus106 prior to engine shutdown. Alternatively, the controller 102 canreceive a request to initiate bowed rotor mitigation on the aircraftcommunication bus 106.

A motoring system 108 is operable to drive rotation of a starting spool(e.g., high speed spool 32) of the gas turbine engine 10 of FIG. 1.Either or both channels of controller 102 can alternate on and offcommands to an electromechanical device 110 coupled to a discretestarter valve 116A to achieve a partially open position of the discretestarter valve 116A to control a flow from a starter air supply 114 (alsoreferred to as air supply 114) through a transfer duct 118 to an airturbine starter 120 (also referred to as starter 120 or pneumaticstarter motor 120) to drive rotation of a starting spool of the gasturbine engine 10 below an engine idle speed. The air supply 114 (alsoreferred to as starter air supply 114) can be provided by any knownsource of compressed air, such as an auxiliary power unit or groundcart.

The controller 102 can monitor a speed sensor, such as speed pickup 122that may sense the speed of the engine rotor through its connection to agearbox 124 which is in turn connected to the high speed spool 32 viatower shaft 55 (e.g., rotational speed of high speed spool 32) or anyother such sensor for detecting or determining the speed of the gasturbine engine 10 of FIG. 1. The starter 120 may be coupled to thegearbox 124 of the gas turbine engine 10 of FIG. 1 directly or through atransmission such as a clutch system (not depicted). The controller 102can establish a control loop with respect to rotor speed to adjustpositioning of the discrete starter valve 116A.

The discrete starter valve 116A is an embodiment of a starter valve thatis designed as an on/off valve which is typically commanded to eitherfully opened or fully closed. However, there is a time lag to achievethe fully open position and the fully closed position. By selectivelyalternating an on-command time with an off-command time through theelectromechanical device 110, intermediate positioning states (i.e.,partially opened/closed) can be achieved. The controller 102 canmodulate the on and off commands (e.g., as a duty cycle using pulsewidth modulation) to the electromechanical device 110 to further openthe discrete starter valve 116A and increase a rotational speed of thestarting spool of the gas turbine engine 10 of FIG. 1. In an embodiment,the electromechanical device 110 has a cycle time defined between anoff-command to an on-command to the off-command that is at most half ofa movement time for the discrete starter valve 116A to transition fromfully closed to fully open. Pneumatic lines 112A and 112B or amechanical linkage (not depicted) can be used to drive the discretestarter valve 116A between the open position and the closed position.The electromechanical device 110 can be a solenoid that positions thediscrete starter valve 116A based on intermittently supplied electricpower as commanded by the controller 102. In an alternate embodiment,the electromechanical device 110 is an electric valve controlling muscleair to adjust the position of the discrete starter valve 116A ascommanded by the controller 102.

In the example of FIG. 2, the engine also includes a vibrationmonitoring system 126. The vibration monitoring system 126 includes atleast one vibration pickup 128, e.g., an accelerometer, operable tomonitor vibration of the gas turbine engine 10 of FIG. 1. Vibrationsignal processing 130 can be performed locally with respect to thevibration pickup 128, within the controller 102, or through a separatevibration processing system, which may be part of an engine healthmonitoring system to acquire vibration data 132. Alternatively, thevibration monitoring system 126 can be omitted in some embodiments.

Similar to FIG. 2, FIG. 3 is a schematic illustration of a startingsystem 100A for the gas turbine engine 10 of FIG. 1 in accordance withanother embodiment. The starting system 100A includes controller 102that controls motoring system 108A, as an alternate embodiment of themotoring system 108 of FIG. 2. Rather than using an electromechanicaldevice 110 coupled to a discrete starter valve 116A to achieve apartially open position of the discrete starter valve 116A of FIG. 2,the motoring system 108A of FIG. 3 uses a variable position startervalve 116B. In FIG. 3, either or both channels of controller 102 canoutput a valve control signal 150 operable to dynamically adjust a valveangle of the variable position starter valve 116A that selectivelyallows a portion of the air supply 114 to pass through the variableposition starter valve 116B and transfer duct 118 to air turbine starter120. The variable position starter valve 116B is a continuous/infinitelyadjustable valve that can hold a commanded valve angle, which may beexpressed in terms of a percentage open/closed and/or an angular value(e.g., degrees or radians). Performance parameters of the variableposition starter valve 116B can be selected to meet dynamic responserequirements of the starting system 100A. For example, in someembodiments, the variable position starter valve 116B has a responserate of 0% to 100% open in less than 40 seconds. In other embodiments,the variable position starter valve 116B has a response rate of 0% to100% open in less than 30 seconds. In further embodiments, the variableposition starter valve 116B has a response rate of 0% to 100% open inless than 20 seconds.

The controller 102 can monitor a valve angle of the variable positionstarter valve 116B using valve angle feedback signals 152 provided toboth channels of controller 102. As one example, in an active/standbyconfiguration, both channels of the controller 102 can use the valveangle feedback signals 152 to track a current valve angle, while onlyone channel designated as an active channel outputs valve control signal150. Upon a failure of the active channel, the standby channel ofcontroller 102 can take over as the active channel to output valvecontrol signal 150. In an alternate embodiment, both channels ofcontroller 102 output all or a portion of a valve angle commandsimultaneously on the valve control signals 150. The controller 102 canestablish an outer control loop with respect to rotor speed and an innercontrol loop with respect to the valve angle of the variable positionstarter valve 116B.

As in the example of FIG. 2, the starting system 100A of FIG. 3 alsoincludes vibration monitoring system 126. The vibration monitoringsystem 126 includes at least one vibration pickup 128, e.g., anaccelerometer, operable to monitor vibration of the gas turbine engine10 of FIG. 1. Vibration signal processing 130 can be performed locallywith respect to the vibration pickup 128, within the controller 102, orthrough a separate vibration processing system, which may be part of anengine health monitoring system to acquire vibration data 132.Alternatively, the vibration monitoring system 126 can be omitted insome embodiments.

FIG. 4 is a block diagram of a system 200 for bowed rotor startmitigation that may control the discrete starter valve 116A of FIG. 2 orthe variable position starter valve 116B of FIG. 3 via control signals210 in accordance with an embodiment. The system 200 may also bereferred to as a bowed rotor start mitigation system. In the example ofFIG. 3, the system 200 includes a mission risk factor model 204 whichmay be part of controller 102.

The mission risk factor model 204 determines at least one inferredengine operating thermal parameter, such as an engine temperature valueor heat state (T_(core)) of the gas turbine engine 10, based on at leastone aircraft-based parameter 202 received on the aircraft communicationbus 106. The at least one aircraft-based parameter 202 is anaircraft-level parameter which is not directly observable by enginesystem sensors coupled to the controller 102. As one example, the atleast one aircraft-based parameter 202 can be a throttle lever angle(TLA) profile indicate of a history of fuel demand for the aircraft. Byobserving the at least one aircraft-based parameter 202 over a period oftime, the mission risk factor model can infer an engine operatingthermal parameter, such as T_(core). For instance, T_(core) can bedetermined by relating flight profile information, e.g., altitude,thrust demand, time, etc., with known performance characteristics of thegas turbine engine 10 of FIG. 1 using a series of computations and/orlook-up table(s) stored in memory of controller 102. The heat state ofthe engine 10 during use or T_(core) is inferred by the mission riskfactor model 204 as the engine 10 is being run and valid instances of atleast one aircraft-based parameter 202 are received on the aircraftcommunication bus 106. The mission risk factor model 204 can inferT_(core) absent sensed values at the engine level.

At engine shutdown, the current or most recently determined heat stateof the engine or T_(core shutdown) of the engine 10 is recorded into DSU104, and the time of the engine shutdown t_(shutdown) is recorded intothe DSU 104. Time values and other parameters may be received onaircraft communication bus 106.

During an engine start sequence or restart sequence, a bowed rotor startrisk model 206 of the controller 102 is provided with the data stored inthe DSU 104, namely T_(core shutdown) and the time of the engineshutdown t_(shutdown). In addition, the bowed rotor start risk model 206is also provided with the time of engine start t_(start). Although themission risk factor model 204 and the bowed rotor start risk model 206are separately depicted, it will be understood that the mission riskfactor model 204 and the bowed rotor start risk model 206 may becombined as a single model.

The bowed rotor start risk model 206 maps core temperature model datainferred from the at least one aircraft-based parameter 202 with timedata to establish a motoring time t_(motoring) as an estimated period ofmotoring to mitigate a bowed rotor of the gas turbine engine 10. Themotoring time t_(motoring) is indicative of a bowed rotor risk parametercomputed by the bowed rotor start risk model 206. The bowed rotor riskparameter may be quantified according to a profile curve 402 selectedfrom a family of curves 404 that align with observed aircraftconditions, such as a longer duration of a higher TLA value, whichincreases turbine bore temperature and the resulting bowed rotor risk asdepicted in the example graph 400 of FIG. 6. For instance, a higher riskof a bowed rotor may result in a longer duration of dry motoring toreduce a temperature gradient prior to starting the gas turbine engine10 of FIG. 1. As will be discussed herein and in one embodiment, anengine start sequence may automatically include a modified startsequence; however, the duration of the modified start sequence prior toa normal start sequence will vary based upon the time periodt_(motoring) that is calculated by the bowed rotor start risk model 206.The motoring time t_(motoring) for predetermined target speed N_(target)of the engine 10 can be calculated as a function of T_(core shutdown),t_(shutdown), and t_(start), (e.g., f (T_(core shutdown), t_(shutdown),and t_(start)), while a target speed N_(target) is a predetermined speedthat can be fixed or vary within a predetermined speed range ofN_(targetMin) to N_(targetMax). In other words, the target speedN_(target) can be the same regardless of the calculated time periodt_(motoring) or may vary within the predetermined speed range ofN_(targetMin) to N_(targetMax). The target speed N_(target) may also bereferred to as a dry motoring mode speed.

Based upon these values (T_(core shutdown), t_(shutdown), and t_(start))the motoring time t_(motoring) at a predetermined target speedN_(target) for the modified start sequence of the engine 10 isdetermined by the bowed rotor start risk model 206. Based upon thecalculated time period t_(motoring) which is calculated as a time to runthe engine 10 at a predetermined target speed N_(target) in order toclear a “bowed condition”. In accordance with an embodiment of thedisclosure, the controller 102 can run through a modified start sequenceupon a start command given to the engine 10 by an operator of the engine10 such as a pilot of an airplane the engine is used with. It beingunderstood that the motoring time t_(motoring) of the modified startsequence may be in a range of 0 seconds to minutes, which, of course,depends on the values of T_(core shutdown), t_(shutdown), and t_(start).

In an alternate embodiment, the modified start sequence may only be runwhen the bowed rotor start risk model 206 has determined that themotoring time t_(motoring) is greater than zero seconds upon receipt ofa start command given to the engine 10. In this embodiment and if thebowed rotor start risk model 206 has determined that t_(motoring) is notgreater than zero seconds, a normal start sequence will be initiatedupon receipt of a start command to the engine 10.

Accordingly and during an engine command start, the bowed rotor startrisk model 206 of the system 200 is automatically referenced wherein thebowed rotor start risk model 206 correlates the elapsed time since thelast engine shutdown time and the shutdown heat state of the engine 10as well as the current start time t_(start) in order to determine theduration of the modified start sequence wherein motoring of the engine10 at a reduced speed N_(target) without fuel and ignition is required.As used herein, motoring of the engine 10 in a modified start sequencerefers to the turning of a starting spool by the starter 120 at areduced speed N_(target) without introduction of fuel and an ignitionsource in order to cool the engine 10 to a point wherein a normal startsequence can be implemented without starting the engine 10 in a bowedrotor state. In other words, cool or ambient air is drawn into theengine 10 while motoring the engine 10 at a reduced speed in order toclear the “bowed rotor” condition, which is referred to as a drymotoring mode.

The bowed rotor start risk model 206 can output the motoring timet_(motoring) to a motoring controller 208. The motoring controller 208uses a dynamic control calculation in order to determine a requiredvalve position of the starter valve 116A, 116B used to supply an airsupply or starter air supply 114 to the engine 10 in order to limit themotoring speed of the engine 10 to the target speed N_(target) due tothe position of the starter valve 116A, 116B. The required valveposition of the starter valve 116A, 116B can be determined based upon anair supply pressure as well as other factors including but not limitedto ambient air temperature of the aircraft, parasitic drag on the engine10 from a variety of engine driven components such as electricgenerators and hydraulic pumps, and other variables such that themotoring controller 208 closes the loop for an engine motoring speedtarget N_(target) for the required amount of time based on the output ofthe bowed rotor start risk model 206. In one embodiment, the dynamiccontrol of the valve position (e.g., open state of the valve (e.g.,fully open, ½ open, ¼ open, etc.) in order to limit the motoring speedof the engine 10) is controlled via duty cycle control (on/off timingusing pulse width modulation) of electromechanical device 110 fordiscrete starter valve 116A.

When the variable position starter valve 116B of FIG. 3 is used, a valveangle 207 can be provided to motoring control 208 based on the valveangle feedback 112 of FIG. 3. A rotor speed N2 (i.e., speed of highspeed spool 32) can be provided to the motoring controller 208 and amitigation monitor 214, where motoring controller 208 and a mitigationmonitor 214 may be part of controller 102. Vibration data 132 can alsobe provided to mitigation monitor 214.

The risk model 206 can determine a bowed rotor risk parameter that isbased on the heat stored (T_(core)) using a mapping function or lookuptable. When not implemented as a fixed rotor speed, the bowed rotor riskparameter can have an associated dry motoring profile defining a targetrotor speed profile over an anticipated amount of time for the motoringcontroller 208 to send control signals 210, such as valve controlsignals 150 for controlling variable position starter valve 116B of FIG.3.

In some embodiments, an anticipated amount of dry motoring time can beused to determine a target rotor speed profile in a dry motoring profilefor the currently observed conditions. As one example, one or morebaseline characteristic curves for the target rotor speed profile can bedefined in tables or according to functions that may be rescaled toalign with the observed conditions. An example of a target rotor speedprofile 1002 is depicted in graph 1000 of FIG. 12 that includes a steepinitial transition portion 1004, followed by a gradually increasingportion 1006, and a late acceleration portion 1008 that increases rotorspeed above a critical rotor speed, through a fuel-on speed and anengine idle speed. The target rotor speed profile 1002 can be rescaledwith respect to time and/or select portions (e.g., portions 1004, 1006,1008) of the target rotor speed profile 1002 can be individually orcollectively rescaled (e.g., slope changes) with respect to time toextend or reduce the total motoring time. The target rotor speed profile1002 may include all positive slope values such that the actual rotorspeed 1010 is driven to essentially increase continuously while bowedrotor start mitigation is active. While the example of FIG. 12 depictsone example of the target rotor speed profile 1002 that can be definedin a dry motoring profile, it will be understood that many variationsare possible in embodiments.

An example of the effects of bowed rotor mitigation are illustrated ingraph 420 of FIG. 7 that depicts a normal or cooled engine start (line432) versus a bowed rotor or mitigated engine start (line 434) inaccordance with one non-limiting embodiment of the disclosure. At point436, a pilot or operator of the engine 10 sets or initiates a startcommand of the engine. At point 438 and after the start command isinitiated, the controller 102, based upon the risk model 206, requiresthe engine to motor at a pre-determined speed (N_(pre-determined) orN_(target)), which is less than a normal idle start speed N2 for a time(t_(determined)). The pre-determined speed (N_(pre-determined) orN_(target)) can be defined within a predetermined speed rangeN_(targetMin) to N_(targetMax) that is used regardless of the calculatedtime period t_(motoring) for homogenizing engine temperatures. The timeperiod t_(determined) is based upon the output of the risk model 206.The determined speed (N_(pre-determined) or N_(target)) is achieved bycontrolling the operational position of starter valve 116A, 116B.Thereafter and at point 440 when the required motoring time (determinedfrom the risk model 206) has been achieved, such that the “bowedcondition” has been cleared a normal start sequence with a normal speedN2 is initiated. Subsequently and at point 442, the idle speed N2 hasbeen achieved. This modified sequence is illustrated in one non-limitingmanner by the dashed line 434 of the graph 420 of FIG. 7. It is, ofcourse, understood that (t_(determined)) may vary depending upon theoutputs of the risk model 206, while N_(pre-determined) or N_(target) isa known value. Of course, in alternative embodiments, the risk model 206may be configured to provide the speed of the engine 10 during amodified start sequence. Still further and as mentioned above, thestarter valve may be dynamically varied based upon the outputs of therisk model 206 as well as the pressure of the air supply 114 in order tolimit the motoring speed of the engine 10 to that of N_(pre-determined)or N_(target) during the clearing of a bowed rotor condition. Line 432illustrates a normal start sequence wherein the time t_(determined) iszero for a modified start as determined by the risk model 206.

The example of FIG. 11 illustrates how a valve angle command 902 can beadjusted between 0 to 100% of a commanded starter valve opening togenerate the actual rotor speed 1010 of FIG. 12. As the actual rotorspeed 1010 tracks to the steep initial transition portion 1004 of thetarget rotor speed profile 1002, the valve angle command 902 transitionsthrough points “a” and “b” to fully open the variable position startervalve 116B. As the slope of the target rotor speed profile 1002 isreduced in the gradually increasing portion 1006, the valve anglecommand 902 is reduced between points “b” and “c” to prevent the actualrotor speed 1010 from overshooting the target rotor speed profile 1002.In some embodiments, decisions to increase or decrease the commandedstarter valve opening is based on monitoring a rate of change of theactual rotor speed 1010 and projecting whether the actual rotor speed1010 will align with the target rotor speed profile 1002 at a futuretime. If it is determined that the actual rotor speed 1010 will notalign with the target rotor speed profile 1002 at a future time, thenthe valve angle of the variable position starter valve 116B is adjusted(e.g., increase or decrease the valve angle command 902) at acorresponding time. In the example of FIGS. 11 and 12, the valve anglecommand 902 oscillates with a gradually reduced amplitude between points“c”, “d”, and “e” as the actual rotor speed 1010 tracks to the targetrotor speed profile 1002 through the gradually increasing portion 1006.As dry motoring continues, the overall homogenization of the engine 10increases, which allows the actual rotor speed 1010 to safely approachthe critical rotor speed without risking damage. The valve angle commandtransitions from point “e” to point “f” and beyond to further increasethe actual rotor speed 1010 in the late acceleration portion 1008 abovethe critical rotor speed, through a fuel-on speed and an engine idlespeed. By continuously increasing the actual rotor speed 1010 during drymotoring, the bowed rotor condition can be reduced faster than holding aconstant slower speed.

As one example of an aircraft that includes systems as described herein,mission risk factor model 204 may run on controller 102 of the aircraftto track heat stored (T_(core)) in the turbine at the time of engineshutdown. Modeling of potential heat stored in the system may beperformed as a turbine disk metal temperature model in the mission riskfactor model 204. When the aircraft lands, engines typically operate atidle for a cool down period of time, e.g., while taxiing to a finaldestination. When an engine shutdown is detected, model state data canbe logged by the DSU 104 prior to depowering. When the controller 102powers on at a later time and model state data can be retrieved from theDSU 104, and the bowed rotor start risk model 206 can be updated toaccount for the elapsed time. When an engine start is requested, a bowedrotor risk can be assessed with respect to the bowed rotor start riskmodel 206. Extended dry motoring can be performed during an engine startprocess until the bow risk has sufficiently diminished. If a safetycondition is detected by the controller 102, for instance, anotification that maintenance will be performed, an abort signal 220 canbe triggered to halt dry motoring.

In reference to FIGS. 4 and 12, the mitigation monitor 214 of FIG. 4 canoperate in response to receiving a complete indicator 212 to run averification of the bowed rotor mitigation. The mitigation monitor 214can provide mitigation results 216 to the motoring controller 208 andmay provide result metrics 218 to other systems, such a maintenancerequest or indicator. Peak vibrations can be checked by the mitigationmonitor 214 during the start processes to confirm that bowed rotormitigation successfully removed the bowed rotor condition. Themitigation monitor 214 may also run while dry motoring is active todetermine whether adjustments to the dry motoring profile are needed.For example, if a greater amount of vibration is detected than wasexpected, the mitigation monitor 214 can request that the motoringcontroller 208 reduce a slope of the target rotor speed profile 1002 ofFIG. 12 to extend the dry motoring time before driving the actual rotorspeed 1010 of FIG. 12 up to the critical rotor speed. Similarly, if themagnitude of vibration observed by the mitigation monitor 214 is lessthan expected, the mitigation monitor 214 can request that the motoringcontroller 208 increase a slope of the target rotor speed profile 1002of FIG. 12 to reduce the dry motoring time before driving the actualrotor speed 1010 of FIG. 12 up to the critical rotor speed.

FIG. 5 is a flow chart illustrating a method 300 of bowed rotor startmitigation of a gas turbine engine in accordance with an embodiment. Themethod 300 of FIG. 5 is described in reference to FIGS. 1-12 and may beperformed with an alternate order and include additional steps. At block302, the controller 102 receives at least one aircraft-based parameter202 on aircraft communication bus 106. At block 304, the controller 102determines at least one inferred engine operating thermal parameter(e.g., T_(core)) from the at least one aircraft-based parameter 202(e.g., TLA). The at least one inferred engine operating thermalparameter can be selected from data describing a history of the aircraftbefore an engine shutdown.

At block 306, the controller 102 controls the motoring system 108, 108Ato drive rotation of a starting spool of the gas turbine engine 10 belowan engine idle speed based on determining that the at least one inferredengine operating thermal parameter is within a preselected threshold.The preselected threshold may be a value or range where a greater riskof a bowed rotor condition exists. The motoring system 108, 108A can becontrolled as described above depending on the system implementation.The controller 102 may transmit a notification on the aircraftcommunication bus 106 that the gas turbine engine 10 is beingconditioned for starting to inform aircraft crew and/or a test standoperator. The time since the engine shutdown t_(shutdown) can be used tomodify a time of rotation for dry motoring. If time data is unavailablefrom the DSU 104, dry motoring can be performed for a maximum expectedtime assuming worst case conditions.

If the controller 102 detects a safety condition (which may include datareceived on the aircraft communication bus 106) while dry motoring isactive, the controller 102 can trigger abort signal 220 to abortrotation of the starting spool. A braking torque may be applied to thestarting spool to abort rotation of the starting spool based on theabort signal 220. The controller 102 can also monitor for rotary motionof the shaft using a rotation sensor, such as speed pickup 122 or analternate sensor that detects rotor speed N2. The controller 102 cantrigger removal of power from the motoring system 108, 108A based ondetecting rotary motion of the starting spool above a target speed(e.g., target speed N_(target)). For example, dry motoring may not beneeded if wind or other forces drive rotation of the high speed spool32.

In embodiments, the risk model 206 can be used to determine a motoringtime period t_(motoring) for a start sequence of the gas turbine engine10, where the risk model 206 uses the recorded time of the engineshutdown and the stored heat state of the gas turbine engine 10 at shutdown to determine the motoring time period t_(motoring). The gas turbineengine 10 is motored at a predetermined speed range of N_(targetMin) toN_(targetMax) during the motoring time period, which is less than anormal idle start speed N2. The controller 102 can dynamically vary aposition of starter valve 116A, 116B during the motoring time period inorder to motor the gas turbine engine 10 at the predetermined speedrange of N_(targetMin) to N_(targetMax). The predetermined speed rangeof N_(targetMin) to N_(targetMax) may be tightly controlled to asubstantially constant rotor speed or cover a wider operating rangeaccording to a dry motoring profile.

As one example with respect to FIGS. 3 and 12, the variable positionstarter valve 116B can be initially set to a valve angle of greater than50% open when bowed rotor start mitigation is active. The controller 102can monitor a rate of change of the actual rotor speed 1010, projectwhether the actual rotor speed 1010 will align with the target rotorspeed profile 1002 at a future time based on the rate of change of theactual rotor speed 1010, and adjust a valve angle of the variableposition starter valve 116B based on determining that the actual rotorspeed 1010 will not align with the target rotor speed profile 1002 at afuture time.

Further dynamic updates at runtime can include adjusting a slope of thetarget rotor speed profile 1002 in the dry motoring profile while thebowed rotor start mitigation is active based on determining that avibration level of the gas turbine engine 10 is outside of an expectedrange. Adjusting the slope of the target rotor speed profile 1002 caninclude maintaining a positive slope. Vibration levels may also oralternatively be used to check/confirm successful completion of bowedrotor start mitigation prior to starting the gas turbine engine 10. Forinstance, based on determining that the bowed rotor start mitigation iscomplete, a vibration level of the gas turbine engine 10 can bemonitored while sweeping through a range of rotor speeds including thecritical rotor speed.

In further reference to FIG. 4, the mitigation monitor 214 of FIG. 4 mayreceive a complete indicator 212 from the motoring controller 208 whenthe motoring controller 208 has completed dry motoring, for instance, ifthe motoring time has elapsed. If the mitigation monitor 214 determinesthat the bowed rotor condition still exists based on vibration data 132collected, the motoring controller 208 may restart dry motoring, or amaintenance request or indicator can be triggered along with providingresult metrics 218 for further analysis. Metrics of attempted bowedrotor mitigation can be recorded in the DSU 104 based on determiningthat the attempted bowed rotor mitigation was unsuccessful orincomplete.

Referring now to FIG. 8, a graph 500 illustrating examples of variousvibration level profiles 502 of an engine, such as gas turbine engine 10of FIG. 1 is depicted. The vibration level profiles 502 represent avariety of possible vibration levels observed before and/or afterperforming bowed rotor mitigation. Critical rotor speed 510 is the speedat which a vibration peak is expected due to amplification effects of abowed rotor condition along with other contributions to vibration levelgenerally. A peak vibration 504 at critical rotor speed 510 may be usedto trigger different events. For example, if the peak vibration 504 atcritical rotor speed 510 is below a maintenance action threshold 506,then no further actions may be needed. If the peak vibration 504 atcritical rotor speed 510 is above a damage risk threshold 508, then anurgent maintenance action may be requested such as an engine check. Ifthe peak vibration 504 at critical rotor speed 510 is between themaintenance action threshold 506 and the damage risk threshold 508, thenfurther bowed rotor mitigation actions may be requested, such asextending/restarting dry motoring. In one embodiment, a maintenancerequest is triggered based on the actual vibration level exceedingmaintenance action threshold 506 after completing an attempt of bowedrotor mitigation.

The lowest rotor vibration vs. speed in FIG. 8 (vibration profile 502D)is for a fully homogenized rotor, where mitigation is not necessary(engine parked all night long, for example). The next higher curve showsa mildly bowed rotor and so on. The maintenance action threshold 506 isa threshold for setting a maintenance flag such as requiring atroubleshooting routine of one or more system elements. The damage riskthreshold 508 may be a threshold to trigger a more urgent maintenancerequirement up to and including an engine check. As dry motoring isperformed in embodiments, the gas turbine engine 10 may shift betweenvibration profiles. For instance, when a bow rotor condition is present,the gas turbine engine 10 may experience vibration levels according tovibration profile 502A, if mitigation is not performed. As dry motoringis run, the gas turbine engine 10 may have a vibration profile that isgradually reduced from vibration profile 502A to vibration profile 502Band then vibration profile 502C, for example. By checking the currentvibration level at a corresponding rotor speed with respect to time, thecontroller 102 can determine whether adjustments are needed to extend orreduce the slope of the target rotor speed profile 1002 of FIG. 12depending on an expected rate of bowed rotor reduction. In embodiments,a slope of the target rotor speed profile 1002 in the dry motoringprofile 206 can be adjusted and maintains a positive slope while bowedrotor start mitigation is active based on determining that a vibrationlevel of the gas turbine engine 10 is less than a targeted maximum range512, which may define a safe level of vibration to ensure that no riskof a maintenance action or damage will likely occur if the actual rotorspeed 1010 is increased faster than previously planned.

Accordingly and as mentioned above, it is desirable to detect, preventand/or clear a “bowed rotor” condition in a gas turbine engine that mayoccur after the engine has been shut down. As described herein and inone non-limiting embodiment, the controller 102 may be programmed toautomatically take the necessary measures in order to provide for amodified start sequence without pilot intervention other than theinitial start request. In an exemplary embodiment, the controller 102and/or DSU 104 comprises a microprocessor, microcontroller or otherequivalent processing device capable of executing commands of computerreadable data or program for executing a control algorithm and/oralgorithms that control the start sequence of the gas turbine engine. Inorder to perform the prescribed functions and desired processing, aswell as the computations therefore (e.g., the execution of Fourieranalysis algorithm(s), the control processes prescribed herein, and thelike), the controller 102 and/or DSU 104 may include, but not be limitedto, a processor(s), computer(s), memory, storage, register(s), timing,interrupt(s), communication interfaces, and input/output signalinterfaces, as well as combinations comprising at least one of theforegoing. For example, the controller 102 and/or DSU 104 may includeinput signal filtering to enable accurate sampling and conversion oracquisitions of such signals from communications interfaces. Asdescribed above, exemplary embodiments of the disclosure can beimplemented through computer-implemented processes and apparatuses forpracticing those processes.

While the present disclosure has been described in detail in connectionwith only a limited number of embodiments, it should be readilyunderstood that the present disclosure is not limited to such disclosedembodiments. Rather, the present disclosure can be modified toincorporate any number of variations, alterations, substitutions orequivalent arrangements not heretofore described, but which arecommensurate with the spirit and scope of the present disclosure.Additionally, while various embodiments of the present disclosure havebeen described, it is to be understood that aspects of the presentdisclosure may include only some of the described embodiments.Accordingly, the present disclosure is not to be seen as limited by theforegoing description, but is only limited by the scope of the appendedclaims.

The invention claimed is:
 1. A bowed rotor start mitigation system for agas turbine engine of an aircraft, the bowed rotor start mitigationsystem comprising: a motoring system comprising a starter; and acontroller coupled to the motoring system and an aircraft communicationbus, wherein the controller is configured to perform: determining, by amission risk factor model that models an engine temperature value in thegas turbine engine, at least one inferred engine operating thermalparameter based on at least one aircraft-based parameter received on theaircraft communication bus, wherein the at least one inferred engineoperating thermal parameter is selected from data describing a historyof the aircraft before an engine shutdown and the at least oneaircraft-based parameter is indicative of a history of fuel demand forthe aircraft; and controlling the motoring system to drive rotation of astarting spool of the gas turbine engine below an engine idle speed byadjusting a flow to the starter based on determining that the at leastone inferred engine operating thermal parameter is within a preselectedthreshold.
 2. The bowed rotor start mitigation system as in claim 1,wherein a time since the engine shutdown is used to modify a time ofrotation of the starting spool by the motoring system, and wherein thetime of rotation is determined based on a bowed rotor start risk modelthat maps a plurality of core temperature model data inferred from theat least one aircraft-based parameter with a plurality of time data toestablish a motoring time.
 3. The bowed rotor start mitigation system asin claim 1, wherein the controller transmits a notification on theaircraft communication bus that the gas turbine engine is beingconditioned for starting by controlling the motoring system to driverotation of the starting spool of the gas turbine engine below theengine idle speed.
 4. The bowed rotor start mitigation system as inclaim 1, wherein the controller is configured to abort rotation by themotoring system when a safety condition is detected.
 5. The bowed rotorstart mitigation system as in claim 4, wherein the motoring system isconfigured to apply a braking torque to the starting spool to abortrotation of the starting spool.
 6. The bowed rotor start mitigationsystem as in claim 1, further comprising a rotation sensor for detectingrotary motion of the starting spool, and the controller is furthercoupled to the rotation sensor.
 7. The bowed rotor start mitigationsystem as in claim 6, wherein the controller is configured to removepower from the motoring system based on detecting rotary motion of thestarting spool above a target speed.
 8. The bowed rotor start mitigationsystem as in claim 1, wherein a vibration monitor is used to set amaintenance flag based on detecting a vibration level that exceeds amaintenance action threshold while accelerating the starting spool ofthe gas turbine engine.
 9. The bowed rotor start mitigation system as inclaim 8, wherein the at least one aircraft-based parameter received onthe aircraft communication bus comprises a throttle lever angle profile.10. A method of bowed rotor start mitigation for a gas turbine engine ofan aircraft, the method comprising: determining, by a mission riskfactor model that models an engine temperature value in the gas turbineengine, at least one inferred engine operating thermal parameter basedon at least one aircraft-based parameter received on an aircraftcommunication bus, wherein the at least one inferred engine operatingthermal parameter is selected from data describing a history of theaircraft before an engine shutdown and the at least one aircraft-basedparameter is indicative of a history of fuel demand for the aircraft;and controlling a motoring system to drive rotation of a starting spoolof the gas turbine engine below an engine idle speed by adjusting a flowto a starter of the motoring system based on determining that the atleast one inferred engine operating thermal parameter is within apreselected threshold.
 11. The method as in claim 10, wherein a timesince the engine shutdown is used to modify a time of rotation of thestarting spool by the motoring system, and wherein the time of rotationis determined based on a bowed rotor start risk model that maps aplurality of core temperature model data inferred from the at least oneaircraft-based parameter with a plurality of time data to establish amotoring time.
 12. The method as in claim 10, further comprising:transmitting a notification on the aircraft communication bus that thegas turbine engine is being conditioned for starting by controlling themotoring system to drive rotation of the starting spool of the gasturbine engine below the engine idle speed.
 13. The method as in claim10, further comprising: aborting rotation by the motoring system when asafety condition is detected.
 14. The method as in claim 13, furthercomprising: applying a braking torque to the starting spool to abortrotation of the starting spool.
 15. The method as in claim 10, furthercomprising: detecting rotary motion of the starting spool using arotation sensor.
 16. The method as in claim 15, further comprising:removing power from the motoring system based on detecting rotary motionof the starting spool above a target speed.
 17. The method as in claim10, further comprising: setting a maintenance flag by a vibrationmonitor based on detecting a vibration level that exceeds a maintenanceaction threshold while accelerating the starting spool of the gasturbine engine.
 18. The method as in claim 17, wherein the at least oneaircraft-based parameter received on the aircraft communication buscomprises a throttle lever angle profile.
 19. A bowed rotor startmitigation system for a gas turbine engine of an aircraft, the bowedrotor start mitigation system comprising: a motoring system comprising astarter; and a controller coupled to the motoring system and an aircraftcommunication bus, wherein the controller is configured to perform:determining, by a mission risk factor model that models an enginetemperature value in the gas turbine engine, at least one inferredengine operating thermal parameter based on at least one aircraft-basedparameter received on the aircraft communication bus, wherein the atleast one aircraft-based parameter is indicative of a history of fueldemand for the aircraft; controlling the motoring system to driverotation of a starting spool of the gas turbine engine below an engineidle speed by adjusting a flow to the starter based on determining thatthe at least one inferred engine operating thermal parameter is within apreselected threshold; and aborting rotation by the motoring system whena safety condition is detected.
 20. A bowed rotor start mitigationsystem for a gas turbine engine of an aircraft, the bowed rotor startmitigation system comprising: a motoring system comprising a starter;and a controller coupled to the motoring system and an aircraftcommunication bus, wherein the controller is configured to perform:determining, by a mission risk factor model that models an enginetemperature value in the gas turbine engine, at least one inferredengine operating thermal parameter based on at least one aircraft-basedparameter received on the aircraft communication bus, wherein the atleast one aircraft-based parameter is indicative of a history of fueldemand for the aircraft; and controlling the motoring system to driverotation of a starting spool of the gas turbine engine below an engineidle speed by adjusting a flow to the starter based on determining thatthe at least one inferred engine operating thermal parameter is within apreselected threshold, wherein a vibration monitor is used to set amaintenance flag based on detecting a vibration level that exceeds amaintenance action threshold while accelerating the starting spool ofthe gas turbine engine.